1. Field of Invention
The present invention pertains to monitoring gas species in various gas streams and at varying temperature and other process locations using a multisection laser.
2. Related Art
The use of near-infrared diode lasers for process gas monitoring is highly advantageous due at least in part to a number of chemical species of interest (e.g., CO, CO2, H2O, NO2, etc.) that can be detected with such lasers. In addition, diode laser devices developed for use in telecommunication applications can be readily adaptable for gas detection applications. For example, many of the device requirements for telecommunication applications, such as narrow line width, rapid tuning capability, etc., parallel the requirements for gas detection. Though semiconductor lasers in principle can be manufactured at any wavelength in the NIR region defined by the matrix material, the availability of lasers at specific wavelengths is limited to either custom made lasers or matching telecommunication laser wavelengths with desired absorption features of interest.
The narrow line width and tunability by temperature and current injection are key advantages in using diode laser devices for gas species detection. These features provide the necessary specificity to detect spectral absorption features for a particular chemical species without interferences from surrounding species. However, a main disadvantage associated with applying typical diode lasers for gas monitoring (e.g., a distributed feedback or DFB laser) is the limited tuning range (about 1-3 cm−1) for current injection, which in turn limits the spectral information detectable by a single device. A larger tuning range is possible with temperature control, but this negatively impacts the response time of the measurement. Therefore, in multiple species or multiple absorption line monitoring for control applications, several lasers are typically applied with each laser dedicated to the absorption line or lines accessible by the device.
Species concentration monitoring is performed by propagating a tunable laser beam from a laser through the gas matrix of interest which can be in-situ or provided from an extracted sample. In the case of in-situ monitoring, the process gas temperature must be known to accurately determine the species concentration. The process gas temperature is necessary, because the resultant absorption signal that is recorded is dependent on temperature through the absorption coefficient that appears in the Beer-Lambert law as the product of the line strength and line shape functions. The Beer-Lambert relation describes the resulting absorption of the laser radiation along the measurement path for a single species as follows:Iv=Iv,oe[−S(T)g(v−vo)Nl]  (1)where Iv is the laser intensity at frequency v measured after the beam has propagated across a path l with N absorbing molecules per volume. The incident laser intensity is Iv,o and is referred to as the reference. The amount of laser radiation attenuated is determined by the temperature dependent line strength S(T) and the lineshape function g(v−vo). Inversion of the above equation relates the number density N to the measured laser intensities and known linestrength and pathlength as follows:
                    N        =                              1                                          S                ⁡                                  (                  T                  )                                            ⁢              l                                ·                      ∫                                          ln                ⁡                                  (                                                            I                      vo                                                              I                      v                                                        )                                            ⁢                              ⅆ                v                                                                        (        2        )            With the exception of S(T) in the above equation, the parameters are either measured or known. If the process temperature is relatively constant, then S(T) can be considered constant from either calibration measurements or from validated database values, e.g., values obtained from a HITRAN-HITEMP database.
Direct application of the Beer-Lambert law applies for measurements that monitor the direct absorption of the laser radiation passed through the absorbing medium. Other techniques apply wavelength modulation spectroscopy (WM) or Frequency modulation (FM) spectroscopy to reduce the inherent noise of the laser. In using WM or FM spectroscopy, the recorded spectral data can be either modeled or compared with a library of spectra to obtain the best fit for a given concentration and temperature. In either application (i.e., direct absorption or a modulation technique), processes that undergo temperature variations require a mechanism for obtaining the gas temperature to determine the correct value of the linestrength. In slow temperature varying processes, temperature information can be obtained from the refractory wall temperature by either a thermocouple or optical pyrometer measurements in particle free systems. However, in dynamic processes, the response time of wall temperatures is too slow for an accurate determination of S(T). Therefore, an alternative mechanism for obtaining the temperature is needed for real-time process monitoring.
In U.S. Pat. No. 5,813,767, a multiple laser system is described for waste incineration monitoring. In addition, this patent document describes a method of determining the temperature from the Gaussian component of the recorded spectral line based upon an identification of the Doppler contribution, which depends solely on the temperature. This method is limited in use since extremely high quality data is required to obtain accurate results. For applications on industrial processes that experience high particle densities, temperature gradients, mechanical vibration, rapid variations in temperature and gas composition, and high radiation loads from the process, all of these factors can contribute at different degrees to degrade the quality of the spectrum, thus introducing errors in the results.
In addition, the tuning range of distributed feedback (DFB) lasers limits the level of spectral information that can be monitored by a single device. Extension of the tuning range over several nanometers can be obtained by varying the device temperature, but this method sacrifices the speed at which multiple spectral regions can be monitored due to the time required for the laser to become thermally stable. Alternatively, external cavity lasers such as those commercially available from New Focus (a Division of Bookham, Inc., San Jose, Calif.) operate with a broad tuning range (e.g., a tuning range of 1520-1570 nm with tuning speed of 10 nm/s), but with a sacrifice in speed.
While a single DFB laser system can be used to monitor two or more gas species of interest in a composition, such a single laser system will be limited to set temperature conditions. For example, in a CO monitoring system using a DFB laser near 1560 cm−1 for combustion process monitoring, the concentration of H2O within the composition must also be determined due to the interference that occurs between H2O peaks and CO peaks at a particular region along the absorption spectrum, since the concentration of a species is determined by measuring the absorption peaks corresponding with the species along the absorbance spectrum for a particular wavelength scan. The concentration of H2O must be determined to correct for such interference so as to determine an accurate CO concentration. Such correction becomes particularly important for CO concentrations of 1% or less (where the CO peaks are very tiny and possibly masked by H2O interference peaks). In this example, temperature sensitivity and the level of H2O peak interference decreases as temperatures approach 1090 K. However, for temperatures of 1090 K or greater, the temperature sensitivity becomes a major factor, thus limiting the accuracy of a single DFB laser for determination of CO concentrations.
In applications where multiple point monitoring is desired, the above-described limitations of the DFB laser restrict these monitoring applications to the conditions that match the diode laser-tuning window. For example, using a DFB laser for monitoring CO and H2O at a process location with a characteristic temperature of 1500 K and another point with a characteristic temperature of 800 K would not be feasible. To monitor these two locations would require a minimum of two DFB lasers to capture the spectral data at each location with the desired sensitivity. As the number of measurement points or locations or variations in characteristic measurement conditions (e.g., large fluctuations in temperature) increases for a particular application, the number of DFB lasers required also increases.
Therefore, applications requiring multiple species monitoring, as required in high temperature processes where the temperature is unknown or varies, typically require several DFB lasers to maintain a fast-response time and accuracy. An exemplary multiple species monitoring system is described in U.S. Pat. No. 5,832,842, which includes a plurality of lasers for monitoring the compositions of CO, O2, H2O and HCl in combustion fumes from incineration plants so as to control the fume acidity. Other examples implementing multiple DFB lasers are described in Ebert et al., “Simultaneous Diode-Laser-Based In situ Detection of Multiple Species and Temperature in Gas-Fried Power Plant” Proceedings of the Combustion Institute, Vol. 28, pp. 423-430, 2000 (for monitoring species in a 1 GW gas-fired power plant) and Furlong et al., “Diode-Laser Sensors for Real-Time Control of Temperature and H2O in Pulsed Combustion Systems,” 34th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, AIAA-98-3949, 1998 (for monitoring species in a pulsed waste incinerator).
In such multiple laser systems, the integration of multiple lasers into a system adds to the system complexity and cost by requiring additional wavelength discriminating mechanisms for the different laser wavelengths (e.g., additional supporting electronics, such as temperature and current controllers for each DFB laser in a multiple laser system as well as multiplexing equipment).